This invention is directed to a device for providing polarized light. It is well known that linearly polarized light is required for certain applications. An electric light bulb, which normally emits nonpolarized light, can be used to generate linearly polarized light by using a means of polarization, such as a polarizer or a polarization beam splitter (PBS), a light component can be polarized in any desired direction. Polarized light is used in many fields, such as investigating glass elements for mechanical stresses. Polarized light can also be used to illuminate liquid crystal displays (LCDs), which can also be employed as part of a projection system. Although the present invention is described in the environment of an LCD projection system, it is to be understood that the inventive illumination system is not limited to such applications.
Radiation with polarized light and high luminous power is necessary for projection systems which include liquid crystal modulators, also called light valves, or cells. In order to present color pictures, projection systems split the luminous radiation emitted from a light source using color filters, such as dichromatic mirrors, into three spectral colors red, green and blue light, and modulate the radiation by means of suitable light modulators. The radiation modulated in this way is projected through a system of lenses onto a reflecting or transmitting medium, such as a picture screen, such that the red, green and blue light components are well-adjusted with respect to each other.
A backprojection television is known from SID 89 Digest, pp. 114 through 117, according to an article by J. Kobayashi et al., which uses high-resolution a-Si TFT LCD panels as light modulators. A suitable optical system consisting of several mirrors and projection lenses is also described in this article. In the European document for public inspection EP 258 927 A2 a display system is introduced which is suitable for projection in television and video systems. This system contains individual picture elements (pixels) which are used to modulate light by applying voltages, or currents, to appropriate connections. When light is radiated onto the pixels from one side and the pixels are mapped onto a front projection, or back projection, screen by projection lenses, a visible picture is made by triggering of the individual pixels. The aforementioned document for public inspection also specifies how the efficiency can be improved when using polarized light which is obtained from a lamp through a so-called polarization beam splitter. A light source for polarized light is described in SID 90 Digest, pp. 324 through 326, according to an article by M. Schadt et al., which uses a so-called cholesteric liquid crystal filter. In EP 258 927 A2 a system of illumination is introduced which uses two lamps placed next to each other is described.
The illumination systems known from the state of the art result in high losses, depending on the side (aspect) ratios, because of the geometry of the light beam emitted, which is normally of circular symmetry, and the geometry of the rectangular panel to be illuminated, which normally has an aspect ratio of 4:3 or 16:9.
If polarized light is to be used for the illumination of the light valves, then the means of polarization, for example, polarization beam splitters, can be advantageously designed in such a way that they cause the splitting and deflection of two partial beams. With an arrangement of the invention in which at least two lamps are used has the advantages that with equal illumination of the light valves the individual lamps are less heavily loaded, and if one lamp fails the function of the illumination system is maintained by the other lamp. The use of lamps with lower rated power also has the advantage that the rays are better focussed. This leads to a smaller illuminated area and hence reduced losses.
Moreover, the means which focuses the light from a lamp, can be designed in such a way that it serves as a means of deflection for a partial beam from at last one further lamp. Additional means thus are not needed and the luminous power is increased. The luminous power of the facility according to the invention can be increased even further by using additional means of focusing, such as lens systems. Means which prevent overheating of the system can extend the working life of the system and raise its long-term efficiency. The simultaneous illumination of light valves enables reproduction of color pictures. The illumination system known from the state of the art does not have the reliability required for continuous operation.